a refractory Stirling power conversion system is 0.42 m2/kWe for all power systems above 20 kWe. This power system has a much smaller radiator than the SP-100/su- peralloy Stirling mainly because of its higher temperature. The radiator temperature range is 590 to 640 K, which is 100 K higher than the range of the superalloy version. The shape of the specific area curve for TFE based power systems is driven by the technologies involved. The 5 kWe system has both moderator, which forces the coolant temperature to be held at or below 900 K, and driver fuel. Thus, it has a very low thermal efficiency, 2.1%, and a relatively low radiator temperature range, 840 to 890 K. Between 10 and 50 kWe, driver fuel is no longer used, and so the thermal efficiency increases to 8.3%. Above 100 kWe, the moderator is no longer required and so the primary coolant temperature, and therefore the radiator temperature can be raised. In our calculations the temperature range increases to 990 to 1040 K. The specific areas of the STAR-C and OTR power systems are the next to smallest of all power systems investigated in this report. The small size is a result of good efficiency, 12 to 13%, and a high heat rejection temperature, 1000 K. The system with the smallest specific area for the heat rejection radiator is the SP-100/Rankine system. The small size is achieved through a high efficiency, 20%, and a relatively high heat rejection temperature, 950 K. The specific areas for the power conditioning radiators were based on a radiator temperature of 425 K. The difference between the areas of the ac and de power conditioning curves result from the fact that they have different efficiencies: 91% for direct current and 96% for alternating current. 5 .0 Conclusions Based on our mass and area estimates, there is no compelling reason to choose one nuclear power system technology over another at power levels below 40 kW(e) until requirements become more firmly established. Differences in volume at launch and during operation may be more important than mass differences at the lower power levels. However, differences in power system masses become more significant as the required power increases further and further beyond the 40 to 60 kW(e) range. The near-term system that looks promising from the mass perspective at the higher power levels is the SP-100/Brayton system. In the far-term, substantial mass and area savings could be realized by going to a SP-100/Rankine power system. Until requirements are more firmly defined, especially those dealing with survivability, and power systems are designed to meet these and many other specific satellite integration requirements, comparisons of power system concepts will remain mostly a matter of conjecture because many system attributes, including mass, may be altered dramatically by these requirements. Acknowledgements Mr. Steven L. Hudson and Dr. Richard E. Pepping of Sandia National Laboratories, Mr. David T. Furgal of G2 vanTel, Ltd., and Ms. Barbara I.
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